This is a mechanical device provided in the brake rigging, and forms part of the pull rod, for automatic adjustment of the clearance between the brake blocks and wheels in the brake rigging. This automatically operates to shorten or lengthen the length of the pull rod, to adjust the excess or less slack in the brake rigging or brake block clearance.

This helps to maintain the clearance between the brake block and the wheels to a predetermined constant value always, thereby maintaining the piston stroke of the brake cylinder constant. This, in turn, always maintains constant brake power for the wagon or coach on the run.

Main parts of the SAB:
Control “A” Dimension:

This is the distance between the slack adjuster barrel and the control rod head, measured when the brake is in fully released condition.This is called as ‘Control Dimension because this is the pre-determined dimension, according to which the slack adjuster pays-out /takes-up the slack in the brake rigging.

The control rod ‘A’ dimension for the Different Rolling stock is given below:
Coaching stock:
  • 16+ 2/ -0mm for 13 T Bogie stock
  • 22+ 2/ -0 mm for 16.25 T bogie stock
  • 22+ 4/-0 mm for Air Brake Stock (HS)
Goods Stock:
  • 50 mm for VB Stock
  • 70 mm for Airbrake stock
  • 27 mm for BOBRN
‘e’ Dimension:

This is the distance between the end of the protective sleeve of the screw and the grooved mark on the screw rod when the brake is fully released.

the total capacity of the slack adjuster available for the adjustment of the brake rigging clearance. This dimension will be 375±25 mm for coaching stock and 555±20 mm for goods stock.

It will decrease as wear takes place on brake blocks, wheels, brake gear pins and bushes due to break applications. And it will be the maximum when all Brake blocks, brake gear pins, and bushes are new and all the wheels are in maximum diameter.

As the ‘e’ dimension decreases and reaches to the minimum due to the wear on the wheel tread, which cannot be made up (worn out brake blocks, brake gear pins, and bushes can be replaced with new ones), manual adjustment shall be done according to the worn out wheel diameter, on the adjusting link of the bogie. This will ensure that sufficient capacity of ‘e’ dimension will be again made available for subsequent adjustments.

Functioning of the SAB:

The SAB is working based on the principle of LIMITING FRICTION. Due to this Limiting friction, the nuts that are provided inside the SAB get rotated automatically, whenever the excessive forces offered due to the incorrect slack acting on them. The rotation of these nuts on the screw rod causes the screw rod to move inward or outward for increasing or decreasing the length of pull rod till the correct adjustment of piston stroke and clearance is obtained.

Adjustment of control ‘A’ dimension:

Assemble the slack adjuster (SAB brake regulator) on the bogie brake rigging. Ensure that the handbrake and brake cylinders and the rigging are in fully released condition and in proper working order. Apply and release the brakes few times and again ensure that the brake rigging is in fully released condition. Check the ‘A’ dimension, if found correct, secure the pins correctly.

  • If found more, disconnect the control rod from its bracket and lengthen it by rotating anti-clockwise.
  • If found less, shorten its length by rotating it clockwise.
  • One full rotation of the control rod will alter the ‘A’ dimension by 2 mm. Fix the control rod in its bracket and apply and release the brakes few times, check ‘A’ dimension, adjust it if required and test. Secure the pin when correct.
  • ‘A’ Dimension to be correctly set to maintain the correct piston stroke and in turn the correct brake power. Rotating the SAB or slack adjuster barrel will not alter the ‘A’ dimension. Once set correctly shall not alter it during service.
Take –Up and Pay-Out Test of SAB:
  • For testing the slack adjuster, it need not be removed from the bogie brake rigging. It can be tested as it is on the bogie, during pit-line examination as follows.
  • Make few brake applications and release and note the piston stroke.
Take-up test:
  • Rotate the barrel anticlockwise 2 or 3 times, to increase the brake block clearances. Apply for the brake and release.
  • Note the higher piston stroke, at first application.
  • Apply and release the brakes. The stroke will be normal (equal to the original piston stroke) after 3 or 4 applications. These shows take up is satisfactory,
  • If not slack adjuster is defective
Pay-out test:
  • Rotate the barrel 2 or 3 times clockwise to decrease the brake block clearance. Apply for the brake and release.
  • Note the short piston stroke at the first application. The stroke will be normal (equal to the original piston stroke) after 3 or 4 applications. These shows
  • Pay-out is satisfactory. If not the slack adjuster (SAB) is defective.